1. Field of the Invention
This invention relates to an engine inlet acoustic barrel designed to attenuate dominant frequencies generated by, for example, a jet aircraft turbo-fan engine.
2. Description of Related Art
Although a jet aircraft engine nacelle inlet appears to be a simple structure, it actually embodies a sophisticated adjunct to power plant performance. While the inner surface of the inlet must be made as smooth as possible for aerodynamic reasons, acoustic considerations demand a porous air passage surface and a cavity of a defined depth behind the air passage surface. As a result, current state-of-the-art construction incorporates multiple cavities behind the air passage surface such as might be created by a honeycomb core. However, the core depth required for optimum acoustic attenuation is not necessarily equal to an optimum core depth required for the sandwich barrel structure to withstand all the aerodynamic and inertial loads to which it is subjected in service.
Current inlet acoustic barrels, including those made from metals or advanced composite materials such as graphite epoxy cloth, and/or tape, are constructed from two or three curved segments that are fastened or spliced together to create a nearly cylindrical shape. The segments are constructed, starting from the inlet's inside air-passage surface, of a permeable inner facesheet, a honeycomb core, and a solid non-permeable back facesheet, and are fastened together along splice lines running in the fore-aft direction. The splice lines involved in such designs are generally made of a non-permeable structural attachment material and have several disadvantages, among them that the splice lines decrease the total active acoustic area. In addition, the presence of splices causes the acoustic modes generated by the fan to be altered to lower order modes, which are difficult to attenuate and thus decrease the efficiency of the liner. Other disadvantages of the splice lines include an increase in the structural weight of the barrel, and stress concentrations at the splice lines.
Examples of composite acoustic jet engine liners are disclosed in U.S. Pat. Nos. 5,025,888, 5,014,815, 4,969,535, 4,840,093, 5,175,401, and 5,151,311, each of which is incorporated herein by reference.
The barrel schematically illustrated in FIG. 1(a) represents a currently existing production barrel 1 of the type disclosed in the above-mentioned patents and patent applications. Barrel 1 includes a splice 2 every 120.degree. . The core structure 3 of barrel 1 in the area of the splice may be tapered or stepped, with facesheets 4 formed to facilitate attachment of two sections 5 and 6 using a splice strap 7 and an intracoastal support 8, as shown in FIG. 1(b), which is typical of fiber reinforced composite construction. Alternatively, the core structure may simply be abruptly terminated, as shown in FIG. 1(c), with attachment facilitated by a structural doubler 9, a densified core 10, structural core splices 11, and optional core splices 12, in addition to splice straps 14 and 15 and intracoastal support 16. This example of an existing barrel is more typical in metallic construction. The total area lost by the splice in both FIG. 1(b) and 1(c) is indicated by arrow 18.
Thus, the conventional inlet acoustic barrels constructed from two or three (and possibly more) nearly equally sized curved structural sandwich panel segments are usually fastened together along splice lines with discrete fasteners, to create a nearly cylindrical shape. This segmental construction presumes that each of the segments, by itself, is designed to support in-plane and bending loads as a curved sandwich plate of, starting from the inlet air-passage surface, the above-mentioned perforated permeable facesheet, a core, possibly in the form of a honeycomb core, and, the solid i.e., non-perforated back facesheet.
The sandwich plates are fastened together along a splice-line running in the fore-aft direction, the construction of the panel-to-panel boundary splice line being of necessity more robust that the interior region of the sandwich. The in-plane and bending loads that are distributed rather uniformly in the field of the curved plane must be transmitted through discretized load paths of the individual fasteners connecting panels to each other and, in some instances, to surrounding structure such as intracoastals, rings, or attachment fittings.
Metallic, as well as traditionally configured, advanced composite sandwich designs adhere to the structural philosophy that all required bending rigidity should be developed by causing the inner and outer facesheets to act as a structural sandwich in differential bending and that the core-material between facesheets should support substantially all the shear loads. Under other circumstances, this usually provides a very efficient structure. However, in the case of acoustic barrels, the perforated facesheet 23 near the air passage surface, which may be covered on either side with another material such as stainless steel, woven wire, or a similar-finely porous material to enhance acoustic performance, serves a dual role. It acts as an element of a tuned Helmholtz resonator for acoustic attenuation, as well as a structural-sandwich facesheet that supports loads in differential bending.
Since the air passage facesheet must be perforated for acoustic performance, its cross-sectional area through its thickness is greatly reduced for the purpose of resisting in-plane loads due to direct stretching or differential bending. As a result, the perforated facesheet thickness must be sized to meet structural demands that may conflict with what might be optimum for maximum acoustic performance. In particular, the perforated facesheet thickness must be increased beyond that needed just to replace the volume of material removed from the holes because of the stress concentrations around each hole. The extra thickness required also tends to reduce acoustic performance, because it increases an acoustic property of the perforate called "mass reactance" which is related to the dynamics of vibrating the slug of air contained in the perforated hole. Since acoustic performance is sensitive to the phenomenon, it would be preferable to size the facesheet thickness based on acoustic considerations without having to make concessions to structural requirements. Generally, a thinner perforated facesheet is preferred.
The preferred embodiments of the invention, on the other hand, use an alternative design philosophy that is closer to pure monocoque construction and which offer significant benefits compared to the current art. The basic feature of a one-piece monocoque is that the primary loads are supported by a single continuous structural shell. While the core and permeable facesheet supports some internal loads by virtue of strain compatibility, they can be thought of as nearly parasitic when incorporated in a monocoque or ring stiffened barrel such as described herein.
The preferred forms of the invention, which will be described in more detail below, obviate all discrete seams and splices, especially in the fore-aft direction. Some inlet constructions call for the use of a microporous air passage layer, while other noise attenuation systems may require only the use of discretely perforated air passage skins. Usually, this choice is dictated by the engine manufacturer who is familiar with the noise characteristics of the engine. In either case, the preferred forms of the invention seek to maintain uniformity of the structure and material associated with the acoustic attenuation system in particular, and the entire structural system in general.
This does not mean that the facesheet porosity may not vary over the entire area in some controlled, predetermined way, or that core thicknesses and cell size must be constant everywhere. In fact, controlled variability may be desirable in certain cases. However, the presence of hardwall regions, i.e., abrupt changes from acoustically treated to acoustically untreated regions, especially fore and aft strips of untreated hardwall areas such as found in splice areas, are to be avoided whether or not the system includes some form of microporous facesheet. The presence of hardwall strips (splices) alters the tones generated by the engine in currently unpredictable ways and reduces the essentially tuned liner's effectiveness to attenuate noise energy at desired frequencies.
Although the advantages of composite material technology in reducing the number of parts in aircraft structures have long been recognized, for example as described in U.S. Pat. No. 4,826,106, the acoustic advantages of such a design, in particular as applied to an aircraft engine nacelle, have not been appreciated. U.S. Pat. No. 4,826,106, for example, proposes the formation of fittings in a jet engine cowling assembly as a one-piece structure. In this patent, however, the term "one-piece" refers to the formation of various parts of the cowling using "judiciously placed graphite/epoxy unidirectional tape layers" to form integrated, seamless structures, but not to the elimination of fore-to-aft splices.